1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as used in medicine for examination of patients. The present invention in particular concerns a method and an MRT apparatus for reduction of Nyquist ghosts (also called N/2 ghosts) that appear upon application of echo-planar imaging sequences (EPI sequences) and impair the image quality (and therewith the diagnosis) to a significant degree.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging modality in medicine and biophysics for over 15 years. In this examination modality the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a precession movement. This precession generates the actual measurement signal in MRT, this measurement signal being acquired by suitable reception coils. The measurement subject can be spatially coded in all three spatial directions by non-inhomogeneous magnetic fields generated by gradient coils.
In one possible method for generation of MRT images, a slice is, for example, initially excited in the z-direction. The coding of the spatial information in the slice ensues by combined phase and frequency coding by means of two orthogonal gradient fields that (in the examination of a slice excited in the z-direction) are generated in the x-direction and y-direction by the gradient coils. The imaging sequence is repeated M times for various values of the phase coding gradients (for example Gy), and the magnetic resonance signal is digitized and stored N times in the presence of the readout gradient Gx for each sequence pass. In this manner a number matrix (matrix in k-space) is generated containing N×M data points. An MR image of the slice in question with a resolution of N×M pixels can be directly reconstructed from this raw data set by a two-dimensional Fourier transformation.
In a technique known as echo-planar imaging (EPI), a number of phase-coded echoes are used to fill the raw data matrix. The basic idea of this technology is to generate a series of echoes in the readout gradient (Gx) after an individual (selective) RF excitation, the echoes being associated with various lines in k-space matrix by a suitable modulation of the phase coding gradients (Gy).
One possible form of the echo-planar pulse sequence is shown in FIG. 1A. After an excitation pulse and a refocusing pulse multiple gradient echoes are generated via a sinusoidally-oscillating frequency coding gradient in the readout direction and phase coding. The phase coding ensues in this representation via small gradient-pulses (blips) in the range of the zero crossings of the oscillating frequency coding gradients and leads in this manner to a serpentine pass through the spatial frequency matrix (k-matrix) as is shown in FIG. 1B.
Despite of many limitations, EPI sequences show a high clinical potential (particularly in functional imaging and in perfussion and diffusion measurements) since movement artifacts (for example due to respiration or pulsing movement of blood or of cerebral fluid) can be drastically reduced due to the extremely short measurement time (MR image acquisition in less than 100 ms).
Unwanted artifacts also occur in EPI imaging that, as what are known as Nyquist ghosts (or also called “N/2 ghosts”), which make an image assessment or image interpretation more difficult in the framework of a diagnosis.
This type of image interference causes the actual image information to be displaced in both directions by half of the phase coding steps and shown a second time as an “image ghost” that even overlaps the actual image in part. An example of such an image afflicted with a Nyquist ghost is shown in FIG. 3.
The cause of Nyquist ghosts is based on all possible forms of asymmetries between the echoes occurring due to positive and negative gradient pulses of the alternating readout gradient pulse train. Such echoes, for example, originate from eddy currents or gradient distortions. Examinations of the dependency of the Nyquist ghosts on protocol parameters have ultimately identified the echo-echo interval of the readout gradient pulse train (and therewith the frequency of the gradient pulse alternation) as a significant parameter. This is also the reason why strong Nyquist ghosts with ghost-to-signal ratios of more than 20% are observed in the proximity of resonance phenomena of the MR scanner (for example given acoustic resonances of the gradient coil or of the cryoshield).
In order to compensate for unavoidable, system-intrinsic asymmetries, data for phase correction are also acquired in addition to the actual image data, with which image data separated into even and odd k-space lines can be corrected before the Fourier transformation. Insofar as the asymmetries occurring in the later readout train are already present in identical form during the phase correction exposure, the Nyquist ghost can be eliminated in the ideal case.
For EPI imaging two strategies for phase correction are prevalent in the prior art:
A) Before the actual image acquisition the entire EPI readout gradient pulse train is initially acquired without phase coding, meaning that the blips are omitted and a dimensional projection of the image subject to be examined is acquired. The information thereby acquired with regard to the displacement of the echo positions and echo phases is subsequently evaluated and used later for correction of the subsequent actual (two-dimensional) image acquisition. This method has the advantage that effects can be detected that only dynamically build up over time during the EPI readout pulse train (for example building resonance phenomena).
A disadvantage of this known technique is that a complete additional exposure that extends (in the worst case doubles) the measurement time is necessary for each measurement. Given a longer acquisition series (for example 1000 images in fMRI, up to approximately 10 minutes) unavoidable asymmetry changes that occur during the acquisition series cannot be reacted to dynamically. Given longer echo times in the framework of such series, an intensified dephasing of the nuclear magnetic resonance measurement signal can appear that ultimately severely impairs the quality of the phase correction data.
B) For each image acquisition, a phase correction, scan is acquired immediately after the -excitation pulse but still before the actual (diagnostic) image data acquisition.
The phase correction scan can be an acquisition with at least two adjacent half-waves (with a positive gradient amplitude and a negative gradient amplitude), but typically with three half-waves (+−+ or −+− in order to be able to take into account B0 offset dephasing effects). The information to be evaluated is the displacement both of the position of the echo and the phase of the echo and is used (for example averaged) for correction of the positive and negative gradient pulses of the actual, immediately-following image acquisition. An advantage of this method is that changes can be dynamically reacted to given longer acquisition series and essentially no additional measurement time is incurred (given a readout frequency of approximately 1000 Hz, the duration of each phase correction scan is only approximately 1.5 ms).
A significant disadvantage of this approach is that time-dynamic effects (for example resonance phenomena) that develops during the readout gradient pulse train are not detected.